The cellular industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. The number of cellular users in major metropolitan areas has far exceeded expectations and is outstripping system capacity. Innovative solutions are thus required to meet these increasing capacity needs as well as to maintain high quality service and avoid raising prices. Furthermore, as the number of cellular users increases, the problems associated with co-channel interference become of increased importance.
FIG. 1 illustrates ten cells C1-C10 in a typical cellular mobile radio communication system. Normally, a cellular mobile radio system would be implemented with more than ten cells. However, for the purposes of simplicity, the present invention can be explained using the simplified representation illustrated in FIG. 1. For each cell, C1-C10, there is a base station B1-B10 with the same reference number as the corresponding cell. FIG. 1 illustrates the base stations as situated in the vicinity of the cell center and having omnidirectional antennas.
FIG. 1 also illustrates nine mobile stations M1-M9 which are movable within a cell and from one cell to another. In a typical cellular radio system, there would normally be more than nine cellular mobile stations. In fact, there are typically many times the number of mobile stations as there are base stations. However, for the purposes of explaining the present invention, the reduced number of mobile stations is sufficient.
Also illustrated in FIG. 1 is a mobile switching center MSC. The mobile switching center MSC illustrated in FIG. 1 is connected to all ten base stations B1-B10 by cables. The mobile switching center MSC is also connected by cables to a fixed switch telephone network or similar fixed network. All cables from the mobile switching center MSC to the base stations B1-B10 and cables to the fixed network are not illustrated.
In addition to the mobile switching center MSC illustrated, there may be additional mobile switching centers connected by cables to base stations other than those illustrated in FIG. 1. Instead of cables, other means, for example, fixed radio links may also be used to connect base stations to mobile switching centers. The mobile switching center MSC, the base stations and the mobile stations are all computer controlled.
In traditional cellular mobile radio systems, as illustrated in FIG. 1, each base station has an omnidirectional or directional antenna for broadcasting signals throughout the area covered by the base station. As a result, signals for particular mobile stations are broadcast throughout the entire coverage area regardless of the relative positions of the mobile stations using the system. In the base station, the transmitter has one power amplifier per carrier frequency. Amplified signals are combined and connected to a common antenna which has a wide azimuth beam. Due to the wide beam width of the common antenna, for example 120 or 360 degrees coverage in azimuth, the antenna gain is low and there is no spatial selectivity to use to reduce interference problems.
More recent techniques have focused on using linear power amplifiers to amplify a combined signal from several carrier frequencies which is then feed to a common antenna. In these systems, the common antenna also has a wide azimuth beam. As a result, these systems also suffer from interference problems.
To overcome these problems, antenna systems have been designed which increase the gain of the antenna while decreasing the interference problems associated with a typical base station. Narrow azimuth beams can be accomplished using an antenna array where each antenna section is connected to its own amplifiers. One such antenna system is described in the U.S. application with Ser. No. 08/253,484, entitled "Microstrip Antenna Array", which is incorporated herein by reference. The disclosed microstrip antenna array uses several beams with narrow beam width to cover the area served by the base station. As a result, the gain of the individual beams can be higher than the typical wide beam used by a traditional antenna. Furthermore, polarization diversity can be used instead of spatial diversity to reduce fading variations and interference problems.
An antenna array is thus a group of similar antennas, or antenna sections, arranged in various configurations with proper amplitude and phase relations in order to give certain desired radiation characteristics. The direction and shape of the narrow antenna beam are determined by weighting each column signal with appropriate phase and amplitude factors. This can for instance be implemented as analog phase shifting, digital beamforming or with a beam forming matrix such as a Butler matrix, or a combination of these features.
There are receiving and transmitting antenna arrays comprising a number of receiving and transmitting antenna sections. The receiving and transmitting antenna sections comprises receiving and transmitting components that can distort the phase and the amplitude of signals. In order to more accurately shape and direct antenna beams and receive information about the exact position of the mobile phones, these transmitting and receiving array antennas need to be accurately calibrated, so that any distortion of phase and amplitude, or time delay, of signals are corrected before transmission and after reception of the signals.
There are several known inventions related to the calibration of antenna arrays. In U.S. Pat. No. 5,412,414 a self calibrating phased array radar is described. The operating part of the transmission and the reception may be calibrated by the addition of a corporate calibration network. The antenna array comprises several antenna sections, each comprising four radiating elements. Each antenna section has an in-built calibration function. The calibration function comprises an exciter which provides a signal for calibration and transmission, a receiver including a phase error sensing circuit referenced to the exciter and a measurement port, and a beamformer. The corporate calibration network has one output for every antenna section.
A disadvantage with this configuration is that each antenna section requires a calibration function of its own, resulting in a large amount of calibration circuits.
In GB-2 285 537 A a method of calibrating an antenna array that receives communication signals is disclosed. Each receiving antenna section is selectively disconnected from the corresponding antenna and is instead connected to a respective tapping of a loop. An RF signal is fed through the loop in two different directions in turns. The resulting amplitude and phase of each receiving antenna section are detected in each case. The product of the signals that have traveled in different directions is constant and hence the phase and amplitude distortion in the calibration cable is corrected.
An disadvantage with this method is that the antennas have to be disconnected while calibrating the receivers resulting in interruption in the traffic.
In U.S. Pat. No. 5,248,982 a method and apparatus for calibrating the reception of phased array antennas is disclosed, similar to the one previously described. Two orthogonal calibration signals are injected into the receiving antenna sections from opposite ends of a calibration cable to eliminate the effects of the calibration cable itself.
In EP 0 713 261 A1 a phased array management system and calibration method are described. The phased array comprises transmitting and receiving phased array antennas that each includes a plurality of antenna sections. Each antenna section comprises a phase adjustment network and an amplitude adjustment network. A probe carrier signal is generated by a probe carrier source. By switching the probe carrier, in time sequence, between multiple antenna sections, the differential amplitude and phase characteristics of each of the antenna sections are determined. Corrective weighting coefficients are generated.
The calibration of an antenna array used in a cellular communication system should preferably be time efficient. Recurrent calibration while the system is running, essentially without disturbing the normal traffic in the communication system would be appreciable.